HELP! SUM by dates selected?

I think I see what you want.  The Service Area Allocation field is a resource and you want to multiply those figures for the number of days selected in the slicer.

Change the slicer to be Date instead of DateHierarchy (from the down caret in the field well) because at the moment it is not reflecting the actual data .

Create two measures,

one: MeasureA to countrows in the Date table (straightforward enough),

two: MeasureB to multiply MeasureA by the SUM of the allocation.

Drag them both on to the table visual.  Select multiple dates in the slicer using CTRL

Let me know how it goes.